#include<stdio.h>
#include<math.h>
int main(void)
{
	/*********Begin*********/
	double a, b, c;
	double s, area;

	scanf("%lf %lf %lf", &a, &b, &c);
	s = (a + b + c) / 2.0;

	area = sqrt(s * (s - a) * (s - b) * (s - c));

	printf("%.3f", area);

	/*********End**********/
	return 0;
}